Tokyo -LRB- CNN -RRB- -- Japan 's Ministry of Health , Labor and Welfare is investigating a report that workers at the damaged Fukushima Daiichi nuclear power plant were told to use lead covers in order to hide unsafe radiation levels , an official said .

The alleged incident happened December 1 , nine months after a major earthquake and tsunami ravaged northern Japan and damaged the plant .

`` We 'll firmly deal with the matter once the practice is confirmed to constitute a violation of any law , '' said the ministry official , who could not be named in line with policy .

An official with the plant 's operator , TEPCO , said the company received a report of the alleged incident Thursday from subcontractor Tokyo Energy & Systems . The report said a second subcontractor , Build-Up , created the lead covers and ordered workers to use them over their dosimeters , pocket-size devices used to detect high radiation levels .

The TEPCO official could also not be named in line with policy .

Tokyo Energy & Systems said in its report that the workers never used the covers , the TEPCO official said . Japan 's Asahi Shimbun newspaper , however , reported Saturday that while some workers refused the orders to use the lead covers , nine others did use them for several hours .

The newspaper 's report cited plant workers , who described the lead covers as fitting snugly over the dosimeters inside the breast pockets of the workers ' protection suits .

TEPCO told CNN it ordered Tokyo Energy & Systems Inc. to conduct an investigation and is awaiting a reply .

CNN 's Junko Ogura contributed to this report .

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The incident allegedly happened at the damaged Fukushima Daiichi plant

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The government and plant operator investigating the report

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Report : Workers were told to covers devices used to detect radiation levels